1. Field of the Invention
This invention relates generally to a system and method for calibrating a heading sensor, such as a yaw-rate sensor, and, more particularly, to a system and method for removing sensor bias errors from a yaw-rate sensor to use the yaw-rate sensor to provide an accurate vehicle heading when GPS signals are not available, where the system and method employ a bias update model to calibrate the sensor using the yaw-rate, a steering wheel angle and a differential wheel speed to identify time windows where the vehicle is traveling relatively straight.
2. Discussion of the Related Art
GPS signals, or other Global Navigation Satellite System (GNSS) signals, can provide accurate positioning and navigation. However, GPS receivers suffer from sky visibility-related limitations, for example, in urban canyons and areas with dense tree cover. Further, GPS signals may suffer from multi-path errors or cross-correlation errors in such areas. Because of existing highly sensitive and fast reacquisition GPS technology, accurate GPS signals become available when sky visibility is temporarily improved for short durations, such as 10-20 seconds, even in less than optimum environments. Therefore, the continuity of GPS technology comes down to maintaining positioning accuracy through GPS outages between GPS available time windows.
Automotive-grade inertial sensors, such as yaw-rate sensors and accelerometers, have highly variable bias and scale characteristics that cause sensor drift that typically makes them un-suitable for navigation and heading determination functions without proper error correction techniques. For example, certain automotive-grade yaw-rate sensors allow up to 2 deg/sec variations for the yaw-rate sensor bias. If such a variability is not corrected, and is allowed for over a period of two minutes, a yaw-rate sensor starting with a bias of 0 deg/sec at zero seconds could reach a bias of 2 deg/sec after 120 seconds. If a linear growth of bias were assumed for simplicity, a heading change derived by integrating yaw-rate sensor signals that is not calibrated would indicate a heading change of 120° only as a result of the variation of the bias.
Inertial sensors can be used in combination with GPS receivers to provide a reasonably accurate vehicle heading, and position if a distance measure, such as vehicle wheel speeds, are available, even when the GPS signals are not available. However, automotive-grade inertial sensors do not typically provide the same level of accuracy as GPS signals. GPS/inertial sensor integrated systems can calibrate the inertial sensors and maintain vehicle heading and position accuracy using GPS signals when the GPS signals are available, and use the calibrated inertial sensors when the GPS signals are not available to maintain a heading and a position solution until the GPS signals become available again.
Known yaw-rate sensor calibration algorithms typically approach bias and scale calibration as a two-step process, and require specific vehicle maneuvers to be performed for the calibration. For example, sensor bias calibration may require the vehicle to be driven in a straight line or be stationary for a known period of time so that the accumulated heading error can be directly estimated as a result of sensor bias error. For scale calibration, the vehicle may be required to be driven through a controlled turn.
U.S. patent application Ser. No. 11/770,898, title GPS-Based In-Vehicle Sensor Calibration Algorithm, filed Jun. 29, 2007, assigned to the assignee of this application and herein incorporated by reference, discloses a system and method for calibrating a heading sensor using GPS signals. The system receives wheel speed or rotation signals, a vehicle odometer reading, GPS signals and yaw-rate signals, and uses the GPS signals to calibrate the heading sensor while the GPS signals are available.
As discussed above, the '898 application calibrates the heading sensor using GPS signals when they are available, so that when the GPS signals are not available the heading sensor will be fairly accurate for some period of time. However, if the GPS signals are unavailable for an extended period of time, then it may be desirable to calibrate the heading sensor when the GPS signals are not available to maintain the accuracy of the heading sensor.